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CN108138282B - Galvanized steel sheet for hot pressing and method for producing hot press-formed article - Google Patents

Galvanized steel sheet for hot pressing and method for producing hot press-formed article Download PDF

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Publication number
CN108138282B
CN108138282B CN201680059107.0A CN201680059107A CN108138282B CN 108138282 B CN108138282 B CN 108138282B CN 201680059107 A CN201680059107 A CN 201680059107A CN 108138282 B CN108138282 B CN 108138282B
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China
Prior art keywords
steel sheet
hot
base steel
galvanized
hot pressing
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Expired - Fee Related
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CN201680059107.0A
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Chinese (zh)
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CN108138282A (en
Inventor
大友亮介
武田实佳子
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Kobe Steel Ltd
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Kobe Steel Ltd
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/20—Deep-drawing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/84—Controlled slow cooling
    • C—CHEMISTRY; METALLURGY
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    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/0257—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment with diffusion of elements, e.g. decarburising, nitriding
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0457—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment with diffusion of elements, e.g. decarburising, nitriding
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment following hot rolling
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
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Abstract

Provided are a galvanized steel sheet which can be used for heating for a short time to suppress LME cracking in a hot pressing step, and a hot-pressing galvanized steel sheet obtained by hot-pressing the galvanized steel sheet. The galvanized steel sheet for hot pressing of the present invention is a galvanized steel sheet for hot pressing, characterized in that internal oxides are present on the base steel sheet side from the interface between the galvanized layer and the base steel sheet, and the base steel sheet contains predetermined C, Si, Mn and Cr, and the balance is iron and unavoidable impurities, and further satisfies the following formula (1). (2 x [ Si ]/28.1+ [ Mn ]/54.9+1.5 x [ Cr ]/52.0) ≥ 0.05 … (1) in the above formula (1), [ Si ] represents the Si content in mass% of the base steel sheet, [ Mn ] represents the Mn content in mass% of the base steel sheet, and [ Cr ] represents the Cr content in mass% of the base steel sheet.

Description

Galvanized steel sheet for hot pressing and method for producing hot press-formed article
Technical Field
The present invention relates to a galvanized steel sheet for hot pressing and a method for producing a hot press-formed article.
Background
In recent years, high-strength steel for vehicle bodies has been used to reduce the weight of automobiles, and steel sheets having a tensile strength of over 980MPa have been used in an increasing range. However, the increase in strength causes problems such as a reduction in the life of the die during part processing and an increase in shape variation due to springback.
Therefore, a working method called hot pressing or hot stamping has been developed, and particularly, a working method has been developed as a part having a tensile strength of 1470MPa or more. In this method, a low-strength steel sheet is heated to a temperature of Ac1 or higher, for example, about 900 ℃ or higher to austenitize the steel sheet before press forming, and then formed in a high-temperature region. This can reduce deformation resistance and spring back, and also can ensure high strength because the steel is quenched simultaneously with forming.
On the other hand, in automobile structural materials, it is necessary to have a sacrificial anti-corrosion effect for side members, cross members, pillar lower portions, and the like of a frame member, which are required to have high corrosion resistance, and cold-worked parts using galvanized steel sheets have been conventionally used. In recent years, however, parts such as the frame side member are also required to be formed from a galvanized steel sheet by a hot stamping process to obtain a high-strength and high-corrosion-resistance part.
However, when hot pressing is applied to a galvanized steel sheet, there is a problem that the molten zinc at a high temperature of 900 ℃ causes Embrittlement of the steel sheet, that is, LME (Liquid Metal Embrittlement), which causes cracks at the time of forming, and lowers impact resistance and fatigue strength required for parts. Therefore, the application of the galvanized steel sheet to hot pressing is not so much performed at present.
As a method for avoiding this problem, for example, patent document 1 discloses providing a ductile layer on a zinc plating. Patent document 1 states that the ductile layer can be used to prevent fine cracks on the surface from being generated from the hardened steel by surface oxidation of the steel strip during heating and/or forming and cooling for austenitizing, thereby dispersing the tension as well. However, the ductility layer that can be formed in patent document 1 is limited to about 10 μm. If the ductile layer is thin, the ductile layer is lost by rapid alloying of the zinc plating and iron in the heating step before hot pressing, and it is considered difficult to sufficiently suppress occurrence of LME cracks during hot pressing.
Patent document 2 discloses a method for hot forming using a galvanized steel sheet, which includes the steps of: 1-preparing a steel strip; 2-coating the steel with a layer of zinc or a zinc alloy; 3-heating the coated steel to a temperature between 300 ℃ and the Ac1 temperature of the steel; 4-cutting a billet from the steel strip after the steps 1, 2 or 3; 5-heating the billet to a temperature higher than the Ac1 temperature of the steel; 6-hot forming the blank into a part; 7-hardening the hot-formed part. In step 3 before hot forming, the method is illustrated in which the amount of liquid zinc during forming is reduced by heating the steel material at 300 ℃ or higher and Ac1 point or lower to form a zinc/iron diffusion layer. However, in this method, after the plating treatment, heating is required to be performed for a longer period of time and in a more complicated manner than in the case of the conventional alloyed hot-dip galvanized steel sheet, and therefore, it is considered that the productivity is lowered and the cost is increased.
The present applicant also proposed, in patent document 3, a technique of suppressing LME by heating for a predetermined time or more before thermoforming. However, in view of productivity and cost which are problems of the hot forming process, further intensive studies for improving productivity such as further shortening the residence time of the heating furnace are required.
Patent document 4 discloses that in a steel sheet product intended for heat treatment, a separate final coating is applied to at least one of the free surfaces of the steel sheet product. The final coating film contains an oxide compound, a nitride compound, a sulfide compound, a sulfate compound, a carbide compound, a carbonate compound, a fluoride compound, a hydrate compound, and a hydroxide compound or a phosphate compound of at least one of base metals. Specifically, a technique is disclosed in which the final coating is applied to the surface of the galvanized steel sheet to improve the heat absorbing ability during heating and to enable heating in a short time. However, in this method, although the austenitizing temperature that can be quenched can be reached in a short time, the effect of suppressing LME cannot be obtained.
[ Prior art documents ]
[ patent document ]
[ patent document 1 ] Japanese Kokai publication 2011-508824
[ patent document 2 ] Japanese Kohyo publication No. 2012-512747
[ patent document 3 ] Japanese patent laid-open No. 2014-159624
[ patent document 4 ] Japanese Kohyo 2014-containing 512457 publication
Disclosure of Invention
The present invention has been made in view of the above circumstances, and an object thereof is to provide a galvanized steel sheet for hot pressing that can be used for heating for a short time to suppress occurrence of LME cracks in a hot pressing step; and a method for producing a hot press-formed article obtained by hot pressing the galvanized steel sheet. Hereinafter, the suppression of the occurrence of LME cracks may be simply referred to as "LME suppression".
The galvanized steel sheet according to the present invention, which can be used to solve the above problems, is a galvanized steel sheet for hot pressing, and has a feature that an internal oxide is present on the base steel sheet side from the interface between a galvanized layer and the base steel sheet, and the base steel sheet contains, in mass%, an oxide
C：0.10～0.5％、
Si：0.50～2.5％、
Mn: 1.0 to 3% and
Cr：0～1.0％，
the balance of iron and inevitable impurities, and further satisfies the following formula (1).
(2×[Si]/28.1+[Mn]/54.9+1.5×[Cr]/52.0)≥0.05…(1)
In the above formula (1), [ Si ] represents the Si content in mass%, [ Mn ] represents the Mn content in mass%, and [ Cr ] represents the Cr content in mass%.
The maximum depth of the internal oxide existing on the base steel sheet side from the interface between the galvanized layer and the base steel sheet is preferably 5 μm or more.
The maximum depth of the oxide existing in the base steel sheet from the interface between the zinc-plated layer and the base steel sheet is a μm, and the amount of zinc attached to each surface is bg/m2In this case, the following formula (2) is preferably satisfied.
a≥0.30×b…(2)
The base steel sheet may further contain at least one of the following elements (I) to (IV) as another element.
(I) In mass%, Al: above 0% and below 0.5%;
(II) in mass%, from B: above 0% and below 0.0050%, Ti: above 0% and below 0.10% and Mo: more than 0% and 1% or less of one or more elements selected from the group consisting of;
(III) at least one element selected from the group consisting of Nb, Zr and V in a total amount of more than 0% and not more than 0.10% in mass%;
(IV) more than one element selected from the group consisting of Cu and Ni, the total amount of which is more than 0% and not more than 1% in mass%.
The present invention also includes a method for producing a hot press-formed article obtained by hot pressing the hot-press galvanized steel sheet.
According to the present invention, the heating time for suppressing LME can be shortened when hot-pressing is performed using a galvanized steel sheet. As a result, hot pressing can be performed with higher productivity using the galvanized steel sheet than in the conventional art, and a hot press-formed product having high strength and high corrosion resistance can be produced.
Drawings
FIG. 1 is a schematic explanatory view showing bending for LME evaluation in examples.
Fig. 2 is a view showing a position where an observation sample is extracted from an L-shaped bent material after bending in the example.
FIG. 3 is a graph illustrating the measured location of LME crack depth in an example.
Detailed Description
The inventors have made extensive studies to solve the above problems. As a result, it has been found that if the internal oxide is present on the base steel sheet side from the interface between the galvanized layer and the base steel sheet of the galvanized steel sheet, and preferably, if the internal oxide is dispersed over a certain depth or more from the interface, the time for suppressing heating of LME can be shortened in the hot pressing step using the galvanized steel sheet.
The internal oxide is an oxide containing at least an oxidizable element such as Si, Mn, and Cr. The internal oxides are observed in the crystal grain boundaries and oxides present in the crystal grains of the base steel sheet, as shown in examples described later.
The presence of the internal oxide in the base steel sheet can shorten the time for suppressing heating of LME, and the mechanism thereof has not been completely elucidated, but the following is considered. That is, a steel sheet in which LME is particularly likely to occur during hot pressing may contain a large amount of liquid zinc during processing. By heating before hot pressing, an alloying reaction of zinc and iron occurs between the galvanized layer and the base steel sheet to form an alloy layer having a high melting point, and the amount of liquid zinc during processing tends to be reduced, and LME tends to be suppressed. The inventors have found that the presence of the internal oxide, which is formed by selectively oxidizing the oxidizable element such as Si, in the base steel sheet promotes the alloying reaction. Therefore, it is presumed that even if the heating before the hot pressing is performed for a short time, the alloy layer is formed to sufficiently reduce the amount of liquid zinc, and the occurrence of LME cracks is suppressed.
The internal oxide is dispersed deeply from the interface between the galvanized layer and the base steel sheet on the base steel sheet side, in other words, the alloying reaction is considered to occur more easily as the region of the base steel sheet after dispersion of the internal oxide is larger. Therefore, in the embodiment of the present invention, the maximum depth of the internal oxide is present on the base steel sheet side from the interface between the galvanized layer and the base steel sheet, and is preferably 5 μm or more. The maximum depth is the maximum depth at which the internal oxide exists in the thickness cross-sectional direction from the interface between the galvanized layer and the base steel sheet, as measured in examples described later. Hereinafter, the "maximum depth at which the internal oxide exists" may be simply referred to as "internal oxidation depth". The internal oxidation depth is more preferably 8 μm or more, and still more preferably 10 μm or more. Considering the manufacturing conditions, the upper limit of the internal oxidation depth is substantially 70 μm.
The degree of the LME inhibition effect required varies depending on the molding conditions and the required rust inhibitive performance, depending on the amount of zinc plating adhered. In either case, however, if the galvanized steel sheet according to the embodiment of the present invention is used, the heating time required for LME suppression can be shortened compared to the conventional one.
Preferably, the internal oxidation depth is set to a depth corresponding to the amount of zinc plating, whereby the LME inhibitory effect can be more reliably exhibited. Specifically, the maximum depth of the internal oxide existing on the base steel sheet side from the interface between the zinc-plated layer and the base steel sheet is a μm, and the amount of zinc deposited is bg/m per surface2In this case, it is preferable that this satisfies the following formula (2).
a≥0.30×b…(2)
The above formula (2) will be explained. When the galvanized layer of the galvanized steel sheet is completely alloyed in the heating process in the hot pressing step, the composition of the obtained alloy plating layer is approximately 70 mass% of Fe and 30 mass% of Zn. In the base steel sheet, if the internal oxide is present to a depth corresponding to the amount of iron in the base steel sheet required for alloying the galvanized layer to the state, it is considered that the LME suppression effect can be exhibited to the maximum. The depth a μm is a unit coating weight bg/m of each surface of zinc2The reason for this is that the amount of Zn having a mass ratio of the thickness of a μm to the Fe layer of 7:3 is equivalent to b ═ 0.3 × b (3.3 × a) g/m2. The above formula (2) is based on such a consideration method. For example, if the amount of the rust preventive agent per surface is 80g/m, the rust preventive agent exhibits sufficient rust preventive properties as a general automobile part2The amount of zinc plating adhesion of (2) can maximally suppress the LME internal oxidation depth of 24 μm. the internal oxidation depth a may be 0.30 × b or more, and the upper limit is not particularly limited, but even if it is much deeper than 0.30 × b, the LME suppression effect is saturated.
In order to form the internal oxide, the base steel sheet of the galvanized steel sheet needs to have a composition of components shown below, and particularly, to satisfy the predetermined formula (1). In addition, as described later, it is recommended to control the winding conditions after hot rolling, among the manufacturing conditions of the galvanized steel sheet.
Composition of base Steel sheet
First, the composition of the base steel sheet of the galvanized steel sheet will be described. Hereinafter, "%" in the component composition means "% by mass".
C：0.10～0.5％
C is an element that contributes to the high strength of the hot-pressed steel sheet, i.e., the hot-pressed product, as a solid solution strengthening element. In order to obtain a desired high strength of 980MPa or more by hot pressing, the lower limit of the C amount is set to 0.10% or more. The lower limit of the amount of C is preferably 0.13% or more, more preferably 0.15% or more, and further preferably 0.17% or more. However, if the amount of C becomes excessive, the weldability of the hot press molded product decreases, so the upper limit thereof is made 0.5% or less. The upper limit of the amount of C is preferably 0.40% or less, more preferably 0.35% or less, and still more preferably 0.30% or less.
Si：0.50～2.5％
Si is an element contributing to improvement of the bonding strength of the spot welded portion of the hot press-formed product. Si also has the effect of preventing tempering in the slow cooling step of hot pressing and maintaining the strength of the hot press-formed article. Further, Si forms retained austenite, and is an element that also contributes to improvement of ductility of the part. In order to effectively exhibit these effects, the lower limit of the amount of Si is set to 0.50% or more. The lower limit of the amount of Si is preferably 0.70% or more, more preferably 0.80% or more, still more preferably 0.90% or more, and still more preferably 1.0% or more. However, if the Si content is excessive, the strength becomes too high, and the rolling load at the time of manufacturing the base steel sheet, that is, the hot-rolled pickled steel sheet or the cold-rolled steel sheet, increases. Further, SiO contained on the surface of the base steel sheet occurs at the time of hot rolling2The scale (2) deteriorates the surface properties of the steel sheet after plating. Therefore, the upper limit of the Si content is 2.5% or less. The upper limit of the amount of Si is preferably 2.3% or less, more preferably 2.1% or less.
Mn：1.0～3％
Mn improves hardenability and is an element useful for suppressing high strength variation in hot press formed articles. Further, Mn is an element that promotes alloying in the alloying treatment of plating described later and contributes to securing the Fe concentration in the plating layer. In order to effectively exhibit such an effect, the lower limit of the Mn amount is set to 1.0% or more. The lower limit of the Mn amount is preferably 1.2% or more, more preferably 1.5% or more, and still more preferably 1.7% or more. On the other hand, if the Mn content becomes excessive, the strength becomes too high, and the rolling load during the production of the base steel sheet increases, so the upper limit thereof is made to be 3% or less. The upper limit of the amount of Mn is preferably 2.8% or less, and more preferably 2.5% or less.
Cr：0～1.0％
The hardenability of the hot press-formed article can be sufficiently ensured by containing the above-mentioned amounts of C and Mn, but Cr may be contained in order to further improve the hardenability. Cr is also an element that can be expected to reduce variation in hardness of hot press molded articles. In order to effectively exhibit these effects, the lower limit of the amount of Cr is preferably 0.01% or more. The lower limit of the amount of Cr is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the amount of Cr is excessive, the above-described effects are saturated and the cost is also increased, so the upper limit thereof is set to 1.0% or less. The upper limit of the amount of Cr is preferably 0.5% or less, more preferably 0.3% or less.
Si, Mn and Cr are contained to improve the mechanical properties and hardenability of the steel. However, these elements are more oxidizable than iron as described above, and tend to be oxidized even in an environment with a low oxygen partial pressure such as iron oxide reduction. That is, these elements contribute to the formation of the internal oxide, and in order to obtain the internal oxide, in the embodiment of the present invention, the contents of Si, Mn, and Cr in the base steel sheet satisfy the following formula (1). The left value of the following formula (1) is hereinafter referred to as the X value.
(2×[Si]/28.1+[Mn]/54.9+1.5×[Cr]/52.0)≥0.05…(1)
In the above formula (1), [ Si ] represents the Si content in mass%, [ Mn ] represents the Mn content in mass%, and [ Cr ] represents the Cr content in mass%.
The value of X is preferably 0.06 or more, and more preferably 0.08 or more. From the viewpoint of toughness of the material, the upper limit of the value of X is about 0.24.
The steel sheet according to the embodiment of the present invention has the above-described components, and the balance is made up of iron and unavoidable impurities such as P, S and N. The P, S and N are preferably suppressed in the following ranges.
P is an element that adversely affects the joint strength of the spot welded portion, and if the amount of P is excessive, P segregates at the final solidification surface of the nugget formed by spot welding, and the nugget becomes brittle, resulting in a reduction in the joint strength. Therefore, the P content is preferably 0.020% or less. More preferably 0.015% or less.
S is also an element that adversely affects the bonding strength of the spot-welded part, similarly to P, and if the amount of S is excessive, grain boundary fracture due to grain boundary segregation in the nugget is promoted, and the bonding strength is reduced. Therefore, the S content is preferably 0.010% or less, and more preferably 0.008% or less.
N is bonded to B to reduce the amount of B dissolved in the steel, thereby adversely affecting the hardenability. When the amount of N is excessive, the amount of nitride deposited increases, and the toughness is adversely affected. Therefore, the upper limit of the amount of N is preferably 0.010% or less. More preferably 0.008% or less. In consideration of the cost in steel production, the amount of N is usually 0.001% or more.
In addition to the above elements, optional elements shown below may be further contained in appropriate amounts as necessary.
Al: above 0% and below 0.5%
Al is an element that can be used for deoxidation, and may be contained by 0.01% or more. However, if the amount of Al is excessive, not only the above effect is saturated, but also inclusions such as alumina increase, and workability deteriorates. Therefore, the upper limit of the amount of Al is preferably 0.5% or less. The upper limit of the amount of Al is more preferably 0.3% or less.
From B: above 0% and below 0.0050%, Ti: above 0% and below 0.10% and Mo: more than 0% and 1% or less of one or more elements selected from the group consisting of
B. Ti and Mo are elements that improve the hardenability of the steel sheet. These elements may be used alone or in combination of two or more. Hereinafter, each element will be described.
In order to improve the hardenability of the steel sheet by B, it is preferable to contain B in an amount of 0.0003% or more. The amount of B is more preferably 0.0005% or more, and still more preferably 0.0010% or more. On the other hand, if the amount of B is more than 0.0050%, coarse borides precipitate in the hot press molded article, and the toughness of the molded article deteriorates, so the amount of B is preferably 0.0050% or less, more preferably 0.0040% or less.
Ti fixes N, which is an element having an action of securing a quenching effect from B. In addition, Ti also has an effect of refining the structure. By making the structure finer, the ductility of the part is improved. In order to sufficiently exhibit such an effect, the amount of Ti is preferably 0.01% or more, and more preferably 0.02% or more. However, since the ductility of the steel sheet deteriorates when the Ti content is excessive, the Ti content is preferably 0.10% or less, and more preferably 0.07% or less.
Mo is an effective element for improving the hardenability of the base steel sheet, and reduction in variation in hardness of hot press formed products can be expected. In order to effectively exhibit such an effect, the Mo amount is preferably 0.01% or more. The Mo content is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the amount is too large, the above-described effects are saturated, and the cost is also increased, so that the upper limit of the Mo amount is preferably 1% or less. The Mo amount is more preferably 0.5% or less, and still more preferably 0.3% or less.
One or more elements selected from the group consisting of Nb, Zr, and V: the total content is higher than 0% and less than 0.10%
Nb, Zr, and V have an effect of refining the structure, and have an effect of improving the ductility of the component by refining the structure. These elements may be used alone or in combination of two or more. In order to effectively exhibit such an effect, the lower limit of the total amount of these elements is preferably 0.01% or more, and more preferably 0.02% or more. The total amount may be a single amount when contained alone, or a total amount when two or more kinds are used in combination. However, if the total amount of these elements is excessive, the effect is saturated and the cost is increased, so that the upper limit is preferably 0.10% or less. More preferably 0.05% or less.
One or more elements selected from the group consisting of Cu and Ni: the total content is higher than 0% and less than 1%
Cu and Ni are elements added as necessary when it is desired to impart delayed fracture resistance to a hot press formed article. These elements may be added alone or in combination of two. In order to effectively exhibit such an effect, the total amount of these elements is preferably 0.01% or more. More preferably 0.05% or more. The total amount may be a single amount when contained alone, or a total amount when two or more are used in combination. However, if the amount of these is too large, it causes surface defects during the production of the steel sheet, and therefore the upper limit is preferably 1% or less, more preferably 0.5% or less.
Method for producing galvanized steel sheet
The galvanized steel sheet according to the embodiment of the present invention can be manufactured by casting a steel satisfying the above-described composition → heating → hot rolling → heat treatment as needed → pickling → cold rolling as needed → zinc plating → alloying as needed. Annealing may also be performed prior to the galvanization process. In order to obtain the above-described structure defined in the embodiment of the present invention, it is recommended to appropriately control at least one of the coiling conditions after hot rolling and the heat treatment conditions when the heat treatment is performed after hot rolling, and the pickling conditions, as described below.
First, steel satisfying the above composition is cast. The heating conditions are not particularly limited, and those generally used can be suitably employed. Preferably at a temperature of from about 1100 to about 1300 ℃.
Next, hot rolling is performed. The hot rolling conditions are not particularly limited, and the conditions generally used, such as temperature in rolling: about 850 to 1200 ℃. However, after completion of hot rolling, particularly after coiling, the temperature profile needs to be controlled in a manner described in detail below. The upper limit of the thickness of the hot-rolled steel sheet obtained is preferably 3.5mm or less. The thickness is preferably 3.0mm or less, more preferably 2.5mm or less, and the lower limit of the thickness is about 0.8 mm.
In order to form a state in which oxides of easily oxidizable elements such as Si, Mn, and Cr are dispersed to a sufficient depth of the surface of the steel sheet, the surface of the steel sheet as a whole of the coil must be kept at a sufficiently high temperature for a long time in a non-oxidizing atmosphere in which iron oxide scale does not grow, from after the coiling to the start of the plating treatment. By thus staying in a non-oxidizing atmosphere at a sufficiently high temperature for a long time, the internal oxides can be formed deep into the base steel sheet, and the LME suppression effect can be sufficiently improved.
Usually, the coiled material is cooled in the atmosphere. The outermost peripheral portion of the coil exposed to the outside air is difficult to form an internal oxide and cannot be used, so that the inside of the coil not exposed to the outside air is used. In the above-mentioned "stay for a long time in a sufficiently high temperature state" after hot rolling, specifically, it is preferable that the whole coil stays at approximately 500 ℃ or more for 2 hours or more. As a means for this, there may be mentioned at least one of: (i) increasing the coiling temperature of coiling after hot rolling; (ii) a means that the cooling speed after coiling is slower is adopted; (iii) coiling and carrying out heat treatment after cooling.
When the winding temperature in the above (i) is increased, the winding temperature is preferably 550 ℃ or higher, more preferably 650 ℃ or higher. Since too high a winding temperature takes time to cool, the upper limit of the winding temperature is about 750 ℃ or less. Further, as means for making the cooling rate after winding relatively slow in the above (ii), for example, there are mentioned enlarging the size of the coil and performing heat preservation using a heat insulating material or the like.
More specific examples of the method (iii) include a heat treatment in which the film is wound at 650 ℃ or lower, exposed to the outside air, cooled, wound under normal conditions, and then stored in a heating furnace at 500 ℃ or higher in a rolled state for about 2 hours. Specifically, the heat treatment temperature is preferably 500 ℃ or higher, more preferably 600 ℃ or higher, and still more preferably 700 ℃ or higher, as described above. However, since the heat treatment temperature is too high, the steel structure is also austenitized, the internal oxidation state is changed, and a sufficient LME suppression effect cannot be obtained, the upper limit of the heat treatment temperature is preferably 750 ℃ or less. The heat treatment time is preferably 2.0 hours or more, and more preferably 2.5 hours or more. However, since the heat treatment time is long and productivity is deteriorated, the upper limit of the heat treatment time is preferably 6 hours or less.
Then, pickling and, if necessary, cold rolling are performed. The pickling is intended to remove iron scale formed during hot rolling, i.e., a high-temperature oxidation film of iron. In the acid washing step, acid washing is performed for 5 to 300 seconds using hydrochloric acid having a concentration of 5 to 20% and heated to 70 to 90 ℃ as an acid solution. In this case, it is preferable to add an appropriate amount of an acid washing accelerator such as a mercapto group-containing compound and/or an inhibitor such as an amine-based organic compound to hydrochloric acid. In addition, cold rolling is performed when it is necessary to further improve the sheet thickness accuracy. In consideration of productivity of a plant, the cold rolling ratio is preferably controlled to be in a range of about 20 to 70%. The upper limit of the thickness of the cold-rolled steel sheet thus obtained is preferably 2.5mm or less. More preferably 2.0mm or less, and still more preferably 1.8mm or less.
In the pickling step, a part of the internal oxide formed after the coiling is damaged, and therefore, it is desirable to perform the pickling step in a short time. The pickling time may vary depending on the degree of the high-temperature oxidized film to be formed, the acid type, the concentration, the liquid temperature, and the like of the acid solution to be used, but is preferably 40 seconds or less, more preferably 30 seconds or less, and still more preferably 20 seconds or less.
In cold rolling, the internal oxidation depth is also reduced and reduced by the reduction ratio. Therefore, it is recommended to increase the internal oxidation depth or the like in advance by setting the temperature history after the coiling to a high temperature for a long time or the like so that a desired internal oxidation depth can be obtained after the pickling step and the cold rolling, taking into consideration the degree of the internal oxidation depth reduction in the pickling step and the cold rolling in advance.
As described above, the internal oxidation depth is preferably set to a depth corresponding to the amount of zinc to be deposited as a product, whereby the zinc-base steel sheet alloy layer of the plating layer formed by the plating treatment described below is sufficiently formed, and the LME suppression effect can be exhibited to the maximum.
The galvanized steel sheet according to the embodiment of the present invention can be obtained by subjecting a hot-rolled steel sheet or a cold-rolled steel sheet, which is a raw sheet, to a plating treatment. As a method of plating, hot dip galvanizing or electroplating can be used. Furthermore, alloyed galvanized steel sheets may also be used, i.e. after the plating treatment, byFor example, a steel sheet in which the plating layer is alloyed with the iron of the base steel sheet by heating at 470 to 580 ℃ for about 20 seconds to 10 minutes. The amount of zinc to be attached to each surface may be determined in accordance with the rust inhibitive performance required for the parts. Approximately 30 to 200g/m2The corrosion resistance can be exerted. Hereinafter, the "amount of zinc-plated coating on each surface" may be simply referred to as "amount of zinc-plated coating". The plating layer of the part obtained by hot pressing tends to have a somewhat lower corrosion resistance than that before hot pressing, through an alloying reaction between the plating layer and the base steel sheet. Therefore, in the plating step, it is desirable that the amount of zinc plating adhesion be reduced to an amount that compensates for the above-mentioned reduction in rust prevention performance required for the applicable portion.
When the continuous hot dip galvanizing treatment is performed as the plating treatment, the steel sheet is generally annealed before the plating treatment. The purpose of annealing is to reduce the natural oxide film on the outermost surface of the steel sheet for hot pressing and to ensure the wettability of the steel sheet with the plating. In general, a steel sheet containing Si has poor wettability of plating even when annealed, but as described above, if an internal oxide is formed before plating treatment, the wettability of plating is good, and therefore, general conditions can be adopted as annealing conditions. The annealing condition is, for example, a temperature of 600 to 920 ℃ and a reducing atmosphere, and the temperature is maintained for about 20 to 300 seconds. Thereafter, the plating treatment can be performed by cooling the alloy to a temperature close to the galvanizing bath temperature, for example, a temperature range of 420 to 500 ℃.
Conditions of hot pressing
The galvanized steel sheet is hot-pressed to obtain a hot-press molded article. In obtaining the hot press-molded article, the conditions for hot pressing are not limited, and the conditions generally used can be employed. The hot pressing process includes a heating process, a press working process and a cooling process. In order to obtain a steel part having toughness and the like, the following conditions are preferably adopted in each step.
Heating step of hot pressing step
The hot dip galvanized steel sheet is heated in the heating step. The heating temperature is preferably Ac1 point or more, more preferably { Ac1 point + (Ac3 point-Ac 1 point)/4 } DEG C or more, still more preferably { Ac1 point + (Ac3 point-Ac 1 point)/2 } DEG C or more, and still more preferably { Ac1 point + (Ac3 point-Ac 1 point). times. 3/4} DEG C or more. The upper limit of the heating temperature is preferably not more than (Ac3 point +180) ° C, and more preferably not more than (Ac3 point +150) ° C. By limiting the heating temperature, coarsening of the microstructure constituting the steel part can be suppressed, and the ductility and bendability can be improved.
The Ac1 point, the Ac3 point, and the Ms point described later can be calculated from the following formula (1), the following formula (2), and the following formula (3) described in "lesley (レスリー) iron and steel materials chemical" (pill-type corporation, published 5/31/1985, pp 273). In the following formulas (1) to (3), [ ] indicates the content (mass%) of each element in the steel sheet, and the content of an element not contained in the steel sheet may be calculated as 0 mass%.
Ac1 point (. degree. C.) 723-10.7X [ Mn ] -16.9X [ Ni ] + 29.1X [ Si ] + 16.9X [ Cr ] … (1)
Ac3 point (. degree. C.) 910-
Ms point (. degree.C.) 561-
In the heating step, it is not necessary to constantly measure the temperature of the steel sheet, and if the temperature of the steel sheet is measured in advance in a preliminary experiment and conditions necessary for temperature control are controlled, the temperature may not be measured at the time of production of the product. The rate of temperature rise to the highest temperature at the time of heating does not matter. As a heating method, furnace heating, energization heating, induction heating, or the like can be employed.
After the temperature of the steel sheet reaches the heating temperature, the holding time at the heating temperature is a time at which the LME cracking can be suppressed at least as shown in examples described later. According to the present invention, the holding time at the heating temperature can be shortened as compared with the case of using a conventional galvanized steel sheet. On the other hand, from the viewpoint of suppressing the grain growth of austenite and improving the properties such as toughness of the steel part, the upper limit of the holding time is preferably 30 minutes or less, and more preferably 15 minutes or less.
The heating atmosphere is not particularly limited as long as the plating is not ignited. For example, an atmospheric atmosphere is preferable because the formation of an oxide film on the plating surface can suppress ignition, but even an oxidizing atmosphere and a reducing atmosphere may be used as long as the surface is covered with an oxide film.
Press working procedure in hot press working procedure
In the press working step, press working is performed on the steel sheet heated in the heating step. The starting temperature of the press working is not particularly limited. For example, when the heating temperature is equal to or lower than the above heating temperature and equal to or higher than the Ms point, the working can be easily performed, and the load during the press working can be sufficiently reduced. The lower limit of the press working start temperature is more preferably 450 ℃ or more, and still more preferably 500 ℃ or more. The upper limit of the starting temperature of the press working is, for example, 750 ℃ or lower, more preferably 700 ℃ or lower, and still more preferably 650 ℃ or lower.
The molding completion temperature is not critical, and may be in the range of not less than Ms point, not more than Ms point and not less than (Ms point-150) ° C, but is performed under conditions such that sufficient hardness, for example, tensile strength of 1370MPa or more, required as a part can be achieved.
The hot press molding may be performed a plurality of times continuously after the heating, in addition to the case of only one time.
Cooling step in Hot pressing step
Immediately after the heating step, cooling of the steel sheet is started. The cooling method is not particularly limited, and the following methods may be mentioned: a method of holding in a mold, cooling by the mold; cooling with water, oil, spray, or the like; air cooling; or combinations thereof, and the like. Also, cooling herein includes natural cooling.
The cooling rate in the cooling step is not particularly limited. For example, the average cooling rate in the temperature range from the heating temperature to the Ms point may be 2 ℃/sec or more. The average cooling rate is more preferably 5 ℃/sec or more, and still more preferably 7 ℃/sec or more. The average cooling rate is preferably 70 ℃/sec or less, more preferably 60 ℃/sec or less, and still more preferably 50 ℃/sec or less.
Examples of the hot press-formed article obtained by hot pressing the galvanized steel sheet according to the embodiment of the present invention include hot press-formed articles for automobile bodies such as side members, cross members, and pillar lower portions.
[ examples ] A method for producing a compound
The present invention will be described in more detail with reference to the following examples, but it is needless to say that the present invention is not limited to the following examples, and can be appropriately modified and implemented within the scope that can meet the purpose described above and below, and these are included in the technical scope of the present invention.
Example 1
In example 1, the effect on the LME inhibitory effect was confirmed by changing the internal oxidation depth by changing the acid solution immersion time in the pickling step performed after hot rolling.
(1) Production of test specimens
The steel having the chemical composition shown in table 1 was melted and cast to obtain a slab, and then heated to 1200 ℃ and hot-rolled so that the finish rolling temperature was 860 to 920 ℃, and the slab was wound at a winding temperature of 660 to 680 ℃ and cooled. Specifically, a hot-rolled steel sheet having a thickness of 2.4mm is obtained after staying at a temperature range of 500 ℃ or higher for 2 hours or longer at a temperature lower than the coiling temperature.
[ TABLE 1 ]
Figure BDA0001610886780000151
The hot-rolled steel sheet was further subjected to descaling in an acid pickling step and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.4mm as a starting sheet. In the acid washing step, hydrochloric acid having a liquid temperature of 75 ℃ and a concentration of 15% is used as the acid solution, and the immersion time in the acid solution is 10 seconds or 30 seconds. By changing the time of immersion in the acid solution in this manner, the degree of dissolution of the surface of the steel sheet after descaling was adjusted, and a material having a changed internal oxidation depth was produced.
For the cold rolled steel sheet described above, annealing, hot dip galvanizing treatment and alloying treatment are sequentially performed in a continuous line. In the annealing, the galvanizing treatment and the alloying treatment, a crucible capable of performing atmosphere control as a heating and cooling mechanism and a galvanizing bath is prepared, and a laboratory furnace capable of performing the plating treatment and the alloying treatment in a single process is used.
Specifically, the temperature was raised from room temperature at an average temperature raising rate of 8 ℃/sec to a soaking temperature of 800 ℃ and soaked for 120 seconds, and then the steel sheet was cooled from the soaking temperature at an average cooling rate of 3 ℃/sec to 460 ℃. Next, the steel sheet was plated in a molten zinc plating bath containing 0.13 mass% of Al, and after adjusting the amount of zinc plating deposited by gas wiping, the steel sheet was subjected to an alloying treatment by heating at 550 ℃ for 20 seconds to obtain an alloyed hot-dip galvanized steel sheet. In addition, the atmosphere before the cooling from the annealing and after the annealing to the plating is changed to the original atmosphere in order to secure the adhesion of the plating, specifically, the atmosphere is changed to a state of flowing 5% to 18% H2－N2The state of the gas.
(2) Measurement of Zinc adhesion and Fe concentration in Zinc plated layer
The galvanized adhesion amount and the composition of the galvanized layer, particularly the Fe concentration in the galvanized layer, of the produced alloyed hot-dip galvanized steel sheet were measured in the following manner. That is, the galvanized steel sheet was immersed in a solution containing hexamethylenetetramine in 18% hydrochloric acid to dissolve only the plated layer, and the amount of zinc plating deposited was determined from the change in mass before and after dissolution. Then, the dissolved solution was analyzed by ICP (Inductively Coupled Plasma) emission spectrometry using ICPS-7510 (manufactured by Shimadzu corporation) to determine the Fe concentration in the zinc coating layer.
(3) Measurement of internal oxidation depth of galvanized steel sheet
The internal oxidation depth of the galvanized steel sheet is measured by a reflected electron image observed by a Scanning Electron Microscope (SEM) in the vicinity of the surface of the steel sheet. The presence or absence of the internal oxide was determined based on the presence or absence of an oxide which appears dark in a field of view of 1000 times the reflected electron image in the crystal grain boundary and the crystal grain of the base steel sheet in the region on the base steel sheet side with respect to the interface between the plating and the base steel sheet. The internal oxidation depth was measured at 3 visual fields with a magnification of 1000 times, and the maximum depth from the interface between the plating and the base steel sheet to the position where the oxide was observed was measured, and the average value of the 3 visual fields of the maximum depth was used. When the composition of these oxides is analyzed by SEM-EDX (Energy Dispersive X-ray spectrometry), Si, Mn, and Cr in a ratio larger than the average content in steel can be detected in addition to oxygen and Fe existing around the oxides.
(4) Evaluation of LME inhibition
(4-1) Hot pressing
First, the production of a hot press molded article was simulated, and heating and bending were performed as follows. Specifically, the alloyed hot-dip galvanized steel sheet was cut into pieces of 100mm × 50mm in the following raw material dimensions to obtain samples, and the samples were charged into an electric furnace heated to 900 ℃. After the samples were held in the oven for the respective times, the samples were taken out, cooled to a press starting temperature of 700 ℃, and then subjected to bending processing under the following processing conditions as shown in fig. 1. Specifically, as shown in fig. 1, the bending blade 3 is moved in the direction of the white arrow, whereby the blank 4 sandwiched between the spacer 1 and the punch 2 is bent as indicated by the black arrow to obtain a test piece of a dummy part, that is, an L-bend material 11.
Processing conditions
Raw material size: length 100mm x depth 50mm
Gasket pressure: 5 ton of
Clearance, i.e. the distance between the punch and the curved blade: 1.4mm as thick as the plate
Bending r (rp): 2.5mm
Press start temperature: 700 deg.C
Bottom dead center hold time: 10 seconds
(4-2) measurement of LME crack depth
Fig. 2 is a view showing a position of an L-bend material extraction observation sample after the bending process. As shown in fig. 2, an observation test piece 14 was obtained by cutting the L-shaped bent material 11 after the bending process so that the cross section 13 of the bent portion center 12 could be observed. The observation test piece 14 was embedded in a support base material so as to be able to observe the cross section 13, and was polished and then etched with a nital etching solution. Then, the outer side of the cross section, i.e., the surface layer vicinity on the tensile stress generating side formed by bending, was observed by using an FE-SEM (Field Emission-Scanning Electron Microscope, SUPRA35, ZEISS). Further, magnification: 500 times, size of field: 230 μm × 155 μm and the number of fields: 10 pieces. Then, the depth of the crack from the interface between the plated alloy layer and the steel sheet into the base steel sheet side, i.e., LME crack, was measured. Elemental analysis was performed by SEM-EDX at the interface between the plated alloy layer and the steel sheet to obtain boundaries between Zn-detected regions and non-detected regions. The LME crack is not necessarily deepest at the apex of the bend, and is mostly deep from the apex near the planar portion. Therefore, it is necessary to observe the entire area of the bent portion of the cross section. Specifically, the entire area of the bent portion of the cross section 13 of the observation test piece 14 is observed while moving the field of view. When no LME crack is present, 0 is set, and when a plurality of LME cracks are generated, the deepest LME crack depth is set as the deepest LME crack depth in the cross section 13.
The summary of the method for measuring the LME crack depth (LME depth) of the test piece 14 for observation is as follows. As described above, the LME crack depth was measured at the cross section 13 (referred to as "first cross section 13") of the bend center 12. Thereafter, as shown in fig. 3, the first cross section 13 was ground so that a cross section parallel to the first cross section 13 (referred to as "second cross section 13A") was exposed (fig. 3), and the depth of LME crack of this second cross section 13A was measured. Thereafter, the second cross section 13A was polished so that a cross section parallel to the second cross section 13A (referred to as a "third cross section 13B") was exposed, and the depth of LME cracks in the third cross section 13B was measured. The amount of each grinding was several mm. The grinding and measurement were repeated 9 times, and the depth of LME cracks was measured from the first section to the tenth section. That is, measurements of the depth of 10 LME cracks can be obtained. The maximum LME crack depth among the 10 measurements, i.e., the deepest LME crack depth among all measurements, was defined as the "LME depth" of the test piece 14 for observation.
In other words, after the cross section 13 as the first cross section (first cross section) is observed, polishing is performed so that it can be observed along the cross section 13A parallel to the cross section 13 and separated by several mm from the direction perpendicular to the bending direction, as described in fig. 3. Then, in the cross section 13A, the same observation as in the above cross section 13 was performed, and the deepest LME crack depth was determined in the cross section 13A as the second cross section (second cross section). The measurement is also performed in the third section (third section) shown in fig. 3, i.e., the section 13B. The polishing and observation were repeated in this manner, and a total of 10 cross sections were observed. Then, the depth of the deepest LME crack among the 10 cross sections in total was determined as the LME depth.
In the present example, when the depth of LME was 10 μm or less, the occurrence of LME cracks was suppressed and evaluated as pass, and when it was more than 10 μm, the occurrence of LME cracks was not suppressed and evaluated as fail.
As for LME cracking, the heating temperature before hot pressing is increased, and the heating time is prolonged, which tends to be suppressed. In the present invention, whether the above heating temperature for inhibiting LME can be controlled by having an internal oxide under the plating layer of the galvanized steel sheet: the heating time at 900 ℃ was shorter than that of the conventional one, and the LME inhibitory effect was evaluated as an index. Specifically, a plurality of samples were prepared by changing the heating time, i.e., the in-furnace time, and the LME depth was measured to determine the LME cracking as a pass standard: the shortest in-furnace time of LME depth less than 10 μm can be achieved. The results are shown in Table 2.
[ TABLE 2 ]
Figure BDA0001610886780000201
The following is apparent from Table 2. In Table 2, 2 samples having different pickling times were compared. The comparative samples were combined to form such pairs as: test nos. 1 and 2; test Nos. 3 and 4; test Nos. 5 and 6; test Nos. 7 and 8; test nos. 9 and 10; test nos. 11 and 12; test nos. 13 and 14; test Nos. 15 and 16; test Nos. 17 and 18; and test nos. 19 and 20. The difference in the in-furnace time between the 2 samples to be compared is described in "the effect of shortening the heating time required for LME inhibition" in table 2. The "effect of shortening heating time required for LME inhibition" in table 2 indicates how much the in-furnace time of the sample having a short pickling time is shortened based on the in-furnace time of the sample having a long pickling time. As is clear from table 2, the in-furnace time of the sample (sample with internal oxide) having a short pickling time can be shortened as compared with the in-furnace time of the sample (sample without internal oxide) having a long pickling time. That is, it was found that the heating time for suppressing LME can be shortened by making the pickling time relatively short and causing internal oxides to exist under the galvanized layer. Further, from the comparison between test nos. 21 and 22, particularly the result of test No.22, it is found that the steel used does not satisfy the predetermined composition, and therefore, even if the pickling time is shortened, the internal oxidation depth is 0, and the heating time for suppressing LME cannot be shortened.
Example 2
In example 2, the effect on the LME inhibition effect was confirmed by changing the internal oxidation depth particularly by changing the coiling temperature after hot rolling.
A steel having a chemical composition shown in Table 1 was melted and cast to obtain a slab, and then the slab was heated to 1200 ℃ and hot-rolled so that the finish rolling temperature was 860 to 920 ℃, and unlike example 1, the slab was coiled while changing the coiling temperature between 500 to 730 ℃ and was left to cool for 2 hours or more to obtain a hot-rolled steel sheet having a thickness of 2.4 mm.
The hot-rolled steel sheet was further subjected to descaling in a pickling step and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.4mm corresponding to the base steel sheet of the plated steel sheet. In the acid washing step, hydrochloric acid having a liquid temperature of 75 ℃ and a concentration of 15% was used as the acid solution, and the time for immersing the acid solution in the acid solution was fixed to 10 seconds.
For the cold rolled steel sheet described above, annealing and hot dip galvanizing treatment are performed. In addition, some of the samples were subjected to a hot-dip galvanizing treatment and then to an alloying treatment. In the annealing, the galvanizing treatment and the alloying treatment, a crucible capable of performing atmosphere control as a heating and cooling mechanism and a galvanizing bath is prepared, and a laboratory furnace capable of performing the plating treatment and the alloying treatment in a single process is used.
Specifically, the temperature was raised from room temperature at an average temperature raising rate of 8 ℃/sec to a soaking temperature of 800 ℃ and soaked for 1 minute, and then the steel plate was cooled from the soaking temperature at an average cooling rate of 3 ℃/sec to 460 ℃. Next, the steel sheet was plated in a molten zinc plating bath containing 0.13 mass% of Al, and after adjusting the amount of zinc plating deposited by gas wiping, a part of the sample was directly cooled to obtain a zinc-plated steel sheet. The other samples were subjected to the above wiping and then to an alloying treatment by heating at 550 ℃ for 20 seconds to obtain an alloyed hot-dip galvanized steel sheet. The atmosphere in the annealing is an original atmosphere for ensuring the adhesion of the plating, specifically, 5% H is circulated2－N2The alloying treatment is also performed in the same atmosphere in the gas state. Cooling after annealing before plating, and cooling after plating or after alloying by spraying N2And (4) carrying out gas treatment.
The measurement of the galvanized adhesion amount, the Fe concentration in the galvanized layer, the internal oxidation depth of the produced galvanized steel sheet and the evaluation of the LME inhibition effect were carried out in the same manner as in example 1, and in Table 3, (the internal oxidation depth a μm) to 0.3 × (the adhesion amount bg/m per surface of galvanized steel sheet) were obtained so as to satisfy the formula (2) defined in the present invention2) When the value is 0 or more, the internal oxide is sufficiently formed with respect to the amount of zinc plating adhesion, and the evaluation is preferable.
These results are shown in table 3.
[ TABLE 3 ]
Figure BDA0001610886780000231
The following is apparent from Table 3. In Table 3, 2 or 3 samples having different coiling temperatures were compared. The comparative samples were combined into the following groups: test Nos. 1 to 3, test Nos. 4 to 6, test Nos. 7 to 9, test Nos. 10 to 12, test Nos. 13 to 15, test Nos. 16 and 17, test Nos. 18 and 19, test Nos. 20 and 21, and test Nos. 22 and 23. The difference in the in-furnace time between the 2 or 3 samples compared is described in "effect of shortening heating time required for LME inhibition" in table 3. TABLE 3 "LME inhibition requiredThe effect of shortening the required heating time "indicates how much the in-furnace time of the sample having a high hot rolling coiling temperature is shortened based on the in-furnace time of the sample having the lowest hot rolling coiling temperature. As is clear from table 3, the in-furnace time of the sample having a high hot rolling coiling temperature (sample having an internal oxide) can be shortened as compared with the in-furnace time of the sample having the lowest hot rolling coiling temperature (sample having no internal oxide). That is, it was found that the heating time for suppressing LME can be shortened by increasing the coiling temperature after hot rolling to cause the presence of internal oxides under the galvanized layer. Particularly, as in test Nos. 1 to 3, test No.3 and test No.12 among test Nos. 10 to 12, it is found that the depth of internal oxidation is a μm and the zinc plating adhesion amount bg/m2The relationship (2) satisfies the predetermined expression, and the heating time for suppressing LME can be further shortened. In addition, it is found that in order to satisfy the above formula (2), it is recommended to further increase the winding temperature.
Further, it is found that, in the test nos. 1 to 3 and 10 to 12 and the test nos. 4 to 6 and 13 to 15, which are substantially the same for the steel type and the plating adhesion amount, respectively, the alloyed hot-dip galvanized steel sheet is more preferable than the hot-dip galvanized steel sheet from the viewpoint of further shortening the heating time for suppressing the LME.
From the comparison of test Nos. 24 and 25, particularly the result of test No.25, it was found that the heating time for suppressing LME could not be shortened since the steel type J used did not satisfy the predetermined composition, and therefore, even if the coiling temperature was increased, the internal oxidation depth was 0.
Example 3
In example 3, the influence of the LME inhibitory effect was confirmed by changing the depth of internal oxidation by changing the conditions of the heat treatment performed after hot rolling.
A steel having a chemical composition shown in Table 1 was melted and cast to obtain a slab, and then the slab was heated to 1200 ℃ and hot-rolled so that the finish rolling temperature was 860 to 920 ℃ and the slab was wound at a winding temperature of 500 ℃, that is, at a low temperature and then cooled to obtain a hot-rolled steel sheet having a thickness of 2.4 mm. A part of the hot-rolled steel sheet was cut into 200mm X300 mm pieces, and 3 pieces of the cut pieces were stacked, introduced into an electric furnace at 600 ℃ or 700 ℃ in an atmospheric atmosphere for 180 minutes to be heat-treated, taken out of the furnace, and then cooled. Of the 3 steel sheets thus obtained, only the center one was used as a sample after heat treatment, that is, the heat treatment was performed in a state where oxygen in the atmosphere was blocked, that is, in a non-oxidizing atmosphere. The heat-treated sample and the non-heat-treated sample were pickled. Specifically, hydrochloric acid having a liquid temperature of 75 ℃ and a concentration of 15% was used as the acid solution, and the surface iron oxide layer was removed by immersing the substrate in the acid solution for 10 seconds. Thereafter, the steel sheet was cleaned, dried and cold-rolled to produce a cold-rolled steel sheet having a thickness of 1.4 mm.
Using the obtained galvanized steel sheets, the galvanized steel sheets were subjected to measurement of the galvanized adhesion amount, Fe concentration in the galvanized layer, internal oxidation depth and evaluation of LME inhibitory effect in the same manner as in example 1, (internal oxidation depth a μm) to 0.3 × (adhesion amount of galvanized first surface bg/m) were obtained in the same manner as in example 22). These results are shown in table 4.
[ TABLE 4 ]
Figure BDA0001610886780000261
The following is clear from Table 4. In Table 4, 2 or 3 samples different from the conditions of the heat treatment after hot rolling were compared. The comparative samples were combined into the following groups: test Nos. 1 to 3, test Nos. 4 to 6, test Nos. 7 to 9, test Nos. 10 to 12, test Nos. 13 to 15, test Nos. 16 and 17, test Nos. 18 and 19, test Nos. 20 and 21, test Nos. 22 and 23, test Nos. 24 and 25, and test Nos. 26 and 27. The difference in the in-furnace time between the 2 or 3 samples to be compared is described in "the effect of shortening the heating time required for LME inhibition" in table 4. The "effect of shortening heating time required for LME suppression" in Table 4 indicates that the in-furnace time of the sample subjected to the post-hot rolling heat treatment was shortened much by taking the in-furnace time of the sample not subjected to the post-hot rolling heat treatment as a referenceLess. As is clear from table 4, the in-furnace time of the sample subjected to the post-hot rolling heat treatment (sample in which the internal oxide is present) can be shortened as compared with the in-furnace time of the sample not subjected to the post-hot rolling heat treatment (sample in which the internal oxide is not present). That is, it was found that the heating time for suppressing LME can be shortened by performing heat treatment in a non-oxidizing atmosphere after hot rolling to cause internal oxides to exist under the galvanized layer. In particular, it was found that the depth of internal oxidation was a μm and the amount of zinc adhesion bg/m2The relationship (2) satisfies the predetermined expression, and the heating time for suppressing LME can be further shortened. It is also known that, in order to satisfy the above formula (2), it is recommended to perform heat treatment in a non-oxidizing atmosphere at a higher temperature after hot rolling.
Further, when comparing test Nos. 1 to 3 and 10 to 12 and test Nos. 4 to 6 and 13 to 15, which are substantially the same in steel type and plating deposit amount, respectively, it is understood that the alloyed hot-dip galvanized steel sheet is more preferable than the hot-dip galvanized steel sheet from the viewpoint of further shortening the heating time for suppressing the LME.
From the comparison between test nos. 28 and 29, particularly the result of test No.29, it is found that the steel type J used does not satisfy the predetermined composition, and therefore, even if the heat treatment is performed under the recommended conditions, the internal oxidation depth is 0, and the heating time for suppressing LME cannot be shortened.
The disclosure of the present specification includes the following modes.
Mode 1:
a galvanized steel sheet for hot pressing, characterized in that an internal oxide is present from the interface between a zinc coating layer and a base steel sheet toward the base steel sheet, and the base steel sheet contains, in mass%, an oxide
C：0.10～0.5％、
Si：0.50～2.5％、
Mn: 1.0 to 3% and
Cr：0～1.0％，
the balance being iron and unavoidable impurities, and satisfying the following formula (1).
(2×[Si]/28.1+[Mn]/54.9+1.5×[Cr]/52.0)≥0.05…(1)
In the above formula (1), [ Si ] represents the Si content in mass%, [ Mn ] represents the Mn content in mass%, and [ Cr ] represents the Cr content in mass%.
Mode 2:
the hot-press galvanized steel sheet according to aspect 1, wherein a maximum depth of the internal oxide present on the base steel sheet side from an interface between the galvanized layer and the base steel sheet is 5 μm or more.
Mode 3:
the hot-press-use galvanized steel sheet according to mode 1 or 2, wherein the maximum depth of the internal oxide existing on the base steel sheet side from the interface between the galvanized layer and the base steel sheet is a μm, and the galvanized adhesion amount per surface is bg/m2Then, the following formula (2) is satisfied.
a≥0.30×b…(2)
Mode 4:
the galvanized steel sheet for hot pressing according to any one of aspects 1 to 3, wherein the base steel sheet further contains, as other elements, in mass%, Al: above 0% and below 0.5%.
Mode 5:
the galvanized steel sheet for hot pressing according to any one of aspects 1 to 4, wherein the base steel sheet further contains, as other elements, at least one element selected from the group consisting of
B: more than 0% and less than 0.0050%,
Ti: above 0% and below 0.10% and
mo: more than 0% and 1% or less.
Mode 6:
the galvanized steel sheet for hot pressing according to any one of aspects 1 to 5, wherein the base steel sheet further contains, as another element, one or more elements selected from the group consisting of Nb, Zr, and V, and the total content is higher than 0% and 0.10% or less in mass%.
Mode 7:
the galvanized steel sheet for hot pressing according to any one of aspects 1 to 6, wherein the base steel sheet further contains, as another element, one or more elements selected from the group consisting of Cu and Ni, and the total content is higher than 0% and 1% or less by mass%.
Mode 8:
a method for producing a hot press-formed article, which comprises hot-pressing a hot-press galvanized steel sheet according to any one of modes 1 to 7 to obtain a hot press-formed article.
This application is filed in accordance with the priority claims filed on the basis of the Japanese patent application having an application date of 2015, 10/2 and the patent application No. 2015-197226. Patent application No. 2015-.
[ description of symbols ]
1 liner
2 punch
3 curved blade
4 blank
11L curved material
Center of bending part of 12L bending material
13. Section of bend part of 13A, 13B L bending material
14 test piece for observation

Claims (7)

1. A galvanized steel sheet for hot pressing which shortens the heating time for suppressing LME during hot pressing, characterized in that the galvanized steel sheet for hot pressing contains internal oxides on the base steel sheet side from the interface between the galvanized layer and the base steel sheet, and the base steel sheet contains oxides in mass%
C：0.10～0.5％、
Si：0.50～2.5％、
Mn: 1.0 to 3% and
Cr：0～1.0％，
the balance consisting of iron and unavoidable impurities,
further, the following formula (1) is satisfied,
(2×[Si]/28.1+[Mn]/54.9+1.5×[Cr]/52.0)≥0.05…(1)
in the above formula (1), [ Si ] represents the Si content in mass% of the base steel sheet, [ Mn ] represents the Mn content in mass% of the base steel sheet, [ Cr ] represents the Cr content in mass% of the base steel sheet,
the maximum depth of the internal oxide existing on the base steel sheet side from the interface between the zinc-plated layer and the base steel sheet is a μm, and the amount of zinc-plated adhesion per surface is bg/m2When the above-mentioned composition satisfies the following formula (2),
a≥0.30×b…(2)。
2. the galvanized steel sheet for hot pressing according to claim 1, wherein a maximum depth of an internal oxide present on the base steel sheet side from an interface between the galvanized layer and the base steel sheet is 5 μm or more.
3. The galvanized steel sheet for hot pressing according to claim 1, wherein the base steel sheet further contains, as other elements, in mass%: above 0% and below 0.5%.
4. The galvanized steel sheet for hot pressing according to claim 1, wherein the base steel sheet further contains, as another element, at least one element selected from the group consisting of
B: more than 0% and less than 0.0050%,
Ti: above 0% and below 0.10% and
mo: more than 0% and 1% or less.
5. The galvanized steel sheet for hot pressing according to claim 1, wherein the base steel sheet further contains, as other elements, more than 0% in total and 0.10% or less by mass of at least one element selected from the group consisting of Nb, Zr, and V.
6. The galvanized steel sheet for hot pressing according to claim 1, wherein the base steel sheet further contains, as other elements, more than 0% and 1% or less in total of at least one element selected from the group consisting of Cu and Ni in terms of mass%.
7. A method for producing a hot press-formed product, which is obtained by hot pressing the hot-press-use galvanized steel sheet according to any one of claims 1 to 6.
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